As a material of a next generation power semiconductor device, a group-III nitride semiconductor such as a gallium nitride-based (GaN-based) semiconductor is expected. The GaN-based semiconductor has a wide bandgap, compared to silicon (Si). Therefore, a device of the GaN-based semiconductor achieves high breakdown voltage and small loss, compared to the device of Si.
In a transistor of the GaN-based semiconductor, a high electron mobility transistor (HEMT) structure in which two-dimensional electron gas (2DEG) is used as a carrier is generally applied. The general HEMT is a normally-on transistor that is conductive even when voltage is not applied to a gate. There is a problem in which a normally-off transistor that does not become conductive unless otherwise voltage is applied to the gate is hardly achieved.
In a power circuit or the like that handles large scale power such as hundreds to one thousand volts, normally-off operation is required considering safety. Therefore, proposed is a circuit configuration in which the normally-off operation is achieved by cascode-connecting a normally-on GaN-based semiconductor transistor to a normally-off Si transistor.
However, in such a circuit configuration, there maybe problems in which breakdown of device or degradation in characteristic is caused in the case where overvoltage occurs at a connection point between the two transistors.